1. Field of the Invention
This invention relates to monostable multivibrators. In particular, this invention relates to a circuit that functions as a monostable multivibrator and has a constant duty cycle.
2. Description of the Prior Art
The term multivibrator refers to a two-stage amplifier with positive feed-back. A flip-flop is a bistable multivibrator; a "one-shot" switching circuit is a monostable multivibrator; and a free-running oscillator is an astable multivibrator. As the term indicates a monostable multivibrator has one stable state. When a trigger input pulse is applied, the monostable multivibrator switches from its stable to its unstable state where it generates an output pulse and remains in the unstable state for a predetermined time before returning to its original stable state. This feature makes the monostable multivibrator useful in standardizing pulses of random widths or in generating time-delayed pulses. The monostable circuit is somewhat similar to that of a flip-flop except that one of two cross-coupled metworks permits a-c coupling only. This means that the circuit remains in its unstable state only until a reactive component, such as a capacitor, discharges, after which the monostable returns to its stable state and the output pulse generated during the unstable state has a predetermined width.
The duty cycle of a monostable multivibrator is generally defined as the ratio of the time the monostable is in its stable state before being triggered to the sum of the time the monostable is in its stable state plus the time it is in an unstable state, when it generates an output pulse. Previous duty cycles of monostable multivibrators have varied with the input frequency of the trigger pulses that cause the monostable to switch from its stable to its unstable and produce an output pulse. Usually the unstable state is of a fixed time period, while the time period of the stable state is a function of the frequency of the input switching signal. A variable frequency input signal causes a variable duty cycle.
In some applications, it is desirable that the duty cycle remain constant, that is, the duty cycle not vary with the frequency of the input signal. In applications such as electronic ignition systems for internal combustion engines, for example, the duty cycle should remain constant as explained hereafter.
The dwell angle in automobile ignition systems is the number of angular degrees that the distributor shaft rotates while the contact points in the distributor remain closed. At certain points during rotation of the distributor shaft, lobes on the distributor cam open the contact points and trigger the primary side of an ignition coil. The dwell time is the length of time for the distributor shaft to rotate through the dwell angle. The dwell angle is constant, but the dwell time varies with engine speed. However, the ratio of the dwell time to the dwell time plus the time the contact points are held open by the cam lobes, that is, the trigger pulse time, is a constant.
In order to convert conventional point-contact ignition systems in automobile engines to electronic, it is desirable that the timing of the electrical pulses generated to fire the spark plugs be similar to the timing of pulses generated by point-contact ignition systems. Although dwell time in conventional point-contact ignition systems decreases as engine speed increases, the dwell angle remains constant, and the ratio of dwell time to dwell time plus time contact points are open, that is, the trigger pulse time, is a constant. These same characteristics should be present in an electronic ignition system. Dwell time is equivalent to the time the monostable multivibrator is in a stable state. The time the contact points are held open is equivalent to the time period of the output pulse generated by the monostable multivibrator in its unstable state.
A monostable multivibrator is an ideal pulse generator circuit for use in an electronic ignition system, because the former produces pulses of a predetermined voltage level and width in response to incoming trigger signals. For the pulses generated by the monostable multivibrator in an electronic ignition system to function substantially equivalent to those generated by the point-contact ignition system, the duty cycle of the monostable multivibrator must be constant. Unfortunately, the duty cycle of prior-art monostable multivibrators varies with the frequency of the input signal, and such operation is not functionally equivalent to that of the conventional point-contact ignition system. Thus, there is a need for a circuit that generates pulses like a monostable multivibrator and has a constant duty cycle.